Determination of carbon in a fly ash sample through comparison to a reference microwave attenuation and phase shift

ABSTRACT

An apparatus and method to measure the unburnt carbon content of fly ash. A microwave signal is transmitted through or reflected from a fly ash sample and the attenuation or phase shift of the signal received by a detector/detectors is determined with respect to the incident signal and is used to provide a measure of unburnt carbon content. The detectors may be antennae means which also launch the incident signal or may be a completely separate means. A preferred embodiment comprises a microwave oscillator for launching the microwave signal into a fly ash sample in a microwave resonant cavity. Detectors determine the attenuation of phase shift of the received signal with respect to a signal from a reference microwave resonant cavity by the fly ash sample, and a measurement of the unburnt carbon content is obtained.

TECHNICAL FIELD

This invention relates to the measurement of the unburnt carbon contentof fly ash produced by a coal fired boiler.

BACKGROUND ART

In the combustion of pulverised coal for steam generation in coal-firedpower stations there are certain fixed losses determined for example, byplant design, and certain controllable losses caused by operating undernon-ideal conditions. The controllable losses comprise:

(a) losses due to incomplete combustion of both solids and combustiblegases;

(b) losses due to the need for excess air.

In practice the controllable losses show a minimum as a function ofoxygen in the flue gas and it is preferable to operate near thisminimum. One way this can be achieved is by basing control of the boileron the measurement of oxygen and carbon monoxide in flue gas. Most largeboilers today are equipped with oxygen analysers which measure O₂ at onepoint in a duct. A problem with these analysers is that the reading isdrastically distorted by air infiltration into the furnace and in theconvection passages downstream of the burners. Also, as measurements aremade at one point, sampling errors are large.

Carbon monoxide in flue gas stays at very low levels at high excess airand rises as excess air is reduced. Infrared CO analysers are availablewhich direct the IR beam across the stack, thus minimising samplingerrors. However, optimising excess air using CO monitors generallyproduces a large amount of unburnt carbon in the ash, because CO levelsare very low at optimum excess air.

An alternative technique is to base control of the boiler on thedetermination of unburnt carbon in the fly ash. A 500 MW power stationburning black coal of 20% ash will produce about 2500 tonnes/hr fluegas, and 37 tonnes/hr fly ash. The carbon content of this fly ash willbe normally in the range 2-5 wt % although it may contain up to 15 wt %carbon. Typically the fly ash concentration in flue gas is about 20g/m³. Present instruments for the determination of the carbon content ofthe fly ash rely on extracting a sample, typically less than 1 gram,from the duct and analysing this on a batch basis typically at 10-20minute intervals.

One prior art carbon concentration monitor [Rupprecht and PatashnickCo., Inc, NYSERDA Report 86-2, January 1986] is based on a microbalanceand small furnace. The instrument collects a 10-50 mg sample of fly ashfrom the outlet duct of a boiler and determines the unburnt carbon inthis sample from the mass loss after heating at 750° C., thismeasurement cycle being repeated at approximately 15 minute intervals.One disadvantage of this analysis technique is that it is very difficultto collect a representative sample of such small size, and thereforesampling uncertainty significantly limits the accuracy of the unburntcarbon determination. The analysis accuracy for replicate samples inlaboratory tests was approximately ±0.5 wt % at 2.3 wt % carbon.

Another commercially available device [Energy and Environmental ResearchCorporation, 18 Mason, Irvine, Calif., USA; December 1987] for thedetermination of unburnt carbon in fly ash collects an approximately 1gram sample from the duct using an isokinetic sampler and analyses thisfor unburnt carbon content from the measured surface reflectance of thesample. The sample collection and measurement cycle is repeated atapproximately 5 minute intervals. In a plant test of the instrument atthe Nefo power plant, Denmark, the analysis accuracy was approximately±1 wt % at less than 3 wt % carbon and ±0.5 wt % at greater than 3 wt %carbon. The analysis accuracy is limited by sampling uncertainty, due tothe sample size and measuring principle (i.e. surface reflectance) used,and the sensitivity of the reflectance measurement to coal type.

A device based on a measurement of the capacitance of a fly ash filledcapacitor has been proposed for the determination of carbon in fly ashin Australian Patent 562440. In this arrangement ash is taken from anash hopper using a screw conveyor, fed into a measuring chamber into theelectric field established by the electrodes of a capacitor and thechange in capacitance of the capacitor measured, and finally returned tothe ash hopper using a second screw conveyor. The bulk density of theash in the measuring chamber is assumed to be approximately constant,although compensation for variation in the bulk density is possibleusing a weighing device.

A microwave technique has been proposed for simultaneously reducing andmeasuring the carbon content in fly ash in U.S. Pat. No. 4,705,409. Inthis technique ash is taken from an ash hopper and passed through ametallic waveguide. Microwave radiation directed through the guide ispreferentially absorbed by the carbon in the fly ash, and theconcentration of carbon is determined from measuring the temperaturerise of a water wall surrounding the guide. Sufficient microwave poweris injected into the guide to burn the excess carbon in the ash andgenerate a reduced carbon product. One disadvantage of this technique isthat the heat conduction out of the guide, and the associatedtemperature rise in the water wall, is a function of not only the carboncontent of the ash but also the chemical characteristics, temperatureand heat conduction properties of the ash. These factors need to betaken into account in the calibration and operation of the device.

Nuclear measurement of carbon in fly ash has also been investigated[Steward, R. F., ISA Transactions, (3), 1967, 200-207]. In thistechnique carbon concentration is correlated with counts of 4.43 MeVgamma rays produced from carbon atoms by the inelastic scatter ofneutrons. Using this technique in laboratory measurements on 10 kg flyash samples the analysis accuracy is repeated as ±0.5 wt % over therange 2-16 wt % carbon.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a method and apparatus tomeasure the unburnt carbon content in fly ash.

Accordingly, in one aspect this invention consists in an apparatus tomeasure the unburnt carbon content of fly ash comprising means togenerate a microwave signal, transmitter means to launch said microwavesignal for transmission through a fly ash sample, receiver means toreceive a signal passed through the sample and processing means todetermine the attenuation or phase shift of the signal passed throughthe sample with respect to the launched signal and to produce a measureof unburnt carbon content.

In a second aspect this invention consists in an apparatus to measurethe unburnt carbon content of fly ash comprising means to generate amicrowave signal, antennae means to launch a microwave signal into a flyash sample and to receive a reflected signal and processing means todetermine the attenuation or phase shift of the reflected signal withrespect to the launched signal and to produce a measure of unburntcarbon content.

In a third aspect this invention consists in a method of measuring theunburnt carbon content of fly ash comprising the steps of launching amicrowave signal into a fly ash sample, receiving the transmittedsignal, determining the attenuation or phase shift of the receivedsignal with respect to the launched signal and producing a measure ofunburnt carbon content from said attenuation or phase shift.

In a fourth aspect this invention consists in a method of measuring theunburnt carbon content of fly ash comprising the steps of launching amicrowave signal into a fly ash sample, receiving a component of thesignal reflected from the sample, determining the attenuation or phaseshift of the reflected signal with respect to the launched signal andproducing a measure of unburnt carbon content from said attenuation orphase shift.

In one preferred form of the invention separate microwave transmittersand receivers are used. These are provided with suitable antennae, forexample, horns or microstrip radiators in an open system, andcapacitative or inductive probes in waveguides.

In another preferred form of the invention a single transceiver is usedfor transmitting and receiving. This arrangement is particularlyadvantageous where a reflected signal is measured but can also be usedwhere a signal transmitted through the sample is measured by utilising asuitable microwave reflector and effecting a double pass of the sample.

The microwave signal can be generated using any suitable microwaveoscillator. Preferably the frequency of the microwave signal is in therange of from 1 to 20 GHz.

Although the attenuation of the transmitted or reflected microwavesignals has been found to provide a useful measurement of unburnt carbonin fly ash it is presently preferred to use the change in thecharacteristics of a microwave resonant cavity induced by the presenceof a fly ash sample to produce a measure of unburnt carbon.

Accordingly, it is preferred that a measurement chamber in the form of amicrowave resonant cavity receives a fly ash sample and the processingmeans determines from the attenuation or phase shift of the receivedsignal with respect to the launched signal the change in the resonantcavity characteristics induced by the fly ash sample and producestherefrom a measure of unburnt carbon content.

The resonant cavity characteristics determined from the attenuation orphase shift are preferably resonant frequency, transmitted or reflectedpower at the resonant frequency, and Q-factor. These are preferablydetermined from a swept frequency measurement. The presently preferredtechnique utilises a swept frequency measurement of attenuation.

In a preferred technique two microwave resonant cavities are utilised.The fly ash sample is placed in or passed through one microwave resonantcavity and the other provides a reference measurement. In a furtherpreferred technique a single microwave resonant cavity is used toprovide both a reference measurement (made when the cavity does notcontain the sample) and subsequent measurements when the cavity containsthe sample.

The methods and apparatus of this invention can be used to measureunburnt carbon content of collected fly ash samples or of a fly ashsample entrained in the flue gas from a coal fired boiler.

It will be apparent that the method and apparatus of this invention haveseveral advantages over the prior art. The measurements according tothis invention are non-destructive and require no special samplepreparation. The microwave measurement can be completed almostinstantaneously and therefore a continuous measurement of unburnt carboncontent can be provided. Further, the method and apparatus of thisinvention are not limited by sample size and can be used with samplesvarying from a few grams to tens of kilograms. The ability to analyselarge samples allows sampling uncertainty to be reduced and enablesimproved measurement accuracy. The method and apparatus are alsoapplicable to both collected samples and in situ measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of an apparatus to measure unburntcarbon in fly ash according to a first embodiment of this invention;

FIG. 2 is a schematic diagram of the antennae and sample measurementchamber in FIG. 1 for measurement in free space;

FIG. 3 is a schematic diagram of the antennae and sample measurementchamber in FIG. 1 for measurement in a waveguide;

FIG. 4 is a schematic diagram of the antennae and sample measurementchamber in FIG. 1 for measurement in a microwave resonant cavity;

FIG. 5 is a schematic diagram of a further apparatus to measure unburntcarbon content of fly ash according to this invention.

FIG. 6 is a graph showing correlation of (A/W) with wt % carbon formeasurement in a waveguide;

FIG. 7 is a graph showing correlation of (φ/W) with wt % carbon formeasurement in a waveguide;

FIG. 8 is a graph showing correlation of (A/W) with wt % carbon formeasurement in free space;

FIG. 9 is a graph showing correlation of (φ/W) with wt % carbon formeasurement in free space; and

FIG. 10 is a graph showing correlation of change in resonant frequencywith wt % carbon for measurements in a resonant cavity; and

FIG. 11 is a graph showing correlation of (change in 1/Q) x f_(r) withwt % carbon for measurements in a resonant cavity.

MODES FOR CARRYING OUR THE INVENTION

The propagation of an electromagnetic wave (EM) in a dielectric mediumis described by Maxwell's equations,and the complex amplitude given by

    E(1)=E.sub.o exp (-γ1)                               (1)

where 1 is the distance travelled by the EM wave in the dielectricmedium from some reference point where its amplitude was E_(o), and γ isthe propagation constant of the wave given by

    γ=α+jβ                                    (2)

where α and β are the attenuation and phase constants respectively. Fora non-magnetic dielectric medium α and β are given by ##EQU1## whereε_(o) is permittivity of free space, λ_(o) the wavelength is free space,ε' the dielectric constant of the medium and ε" the loss factor of themedium. The attenuation constant α represents the attenuation of the EMwave (in nepers per meter) and the phase constant β represents the phaseshift of the EM wave (in radians per meter).

From equations (3) and (4), it can be seen that the attenuation andphase shift of an EM wave in a dielectric is a function of the complexpermittivity of the medium,

    ε*=ε'-jε"                          (5)

For a multicomponent dielectric medium the complex permittivity may beapproximated by ##EQU2## where v_(i) and ε*_(i) are the volume fractionand complex permittivity of the i^(th) component respectively.

When a plane EM wave is incident upon a dielectric interface, part of itis reflected and part transmitted. For a non-magnetic dielectric in airthe reflection coefficient, R, and transmission coefficient, T, aregiven by ##EQU3## where E_(o), E_(R) and E_(T) are the incident,reflected and transmitted electric field vectors. From equations (3) and(4) it can be seen that the phase shift and attenuation of a transmittedmicrowave signal are functions of the effective complex permittivity ofthe sample given by equation (6). For fly ash the complex permittivityof the unburnt carbon is significantly different from the remainingmatrix which principally comprises oxides of silicon, aluminium andiron. Therefore the measured attenuation and phase shift for fly ash arestrong functions of the unburnt carbon cshift of a reflected signal aretherefore also functions of the unburnt carbon content of the samples.

For a cylindrical microwave resonant cavity the resonant frequency ofthe microwave cavity, f, can be calculated from, ##EQU4## where `n,m,l`refer to the particular resonant mode (and correspond to the number ofelectric field maxima in the standing wave pattern φ, r and zdirections), `a` and `d` are the cavity radius and length respectivelyand ρ is a constant determined for each resonant mode. For a TM₀₁₀resonant cavity, equation (1) reduces to ##EQU5## When a sample withpermittivity ε*=ε'-jε" is placed about the axis of a TM₀₁₀ cavity, andthe sample radius, r<<a, it is found that, the change in the resonantfrequency, Δf, and Q-factor, Δ 1/Q , are related to the dielectricproperties of the sample by, ##EQU6## where V_(S) is the volume fractionof the cavity filled by the sample. Therefore for a constant volumesample, Δf/f is proportional to ε' and Δ 1/Q is proportional to ε". Itfollows that for measurements on fly ash in such a cavity, Δf/f and Δ1/Q are both strong functions of the weight percent unburnt carbon inthe fly ash.

In the method for determining unburnt carbon content of fly ashaccording to one aspect of this invention a microwave signal is directedthrough a fly ash sample using suitable transmitting and receivingantennae and the attenuation and phase shift of the signal due to thefly ash sample are measured. These are normally calculated as thedifference between the attenuation and phase shift determined with thesample and air. To compensate for variation in the density and thicknessof the fly ash sample the phase shift and attenuation can be normalisedto a unit sample mass per unit area. This is not necessary where thevariation in sample density and thickness can be maintained withinacceptable limits by a suitable sample presentation system.

To obtain a measure of unburnt carbon content in terms of weight percent(wt %) the attenuation or phase shift data are correlated with wt %unburnt carbon, determined by standard laboratory analysis, using leastsquares regression and equations of the form:

    wt % unburnt carbon=a.sub.0 +a.sub.1 (φ.sub.c)         (13)

    wt % unburnt carbon=b.sub.0 +b.sub.1 (A.sub.c)             (14)

where φ_(c) and A_(c) are the corrected (compensated for variation insample density and thickness) phase shift and attenuation respectively,and a₀, . . , b₁ are fitting constants. The unburnt carbon content mayalso be determined from a combined measurement of attenuation and phaseshift, independent of variation in sample density and thickness, usingan equation of the form

    wt % unburnt carbon=C.sub.0 +C.sub.1 (φ.sub.m)+C.sub.2 (A.sub.m)(15)

where φ_(m) and A_(m) are the measured phase shift and attenuationrespectively, and C₀, . . , C₂ are fitting constants.

In the method for determining unburnt carbon content of fly ashaccording to another aspect of the invention a microwave signal isdirected at a fly ash sample and the reflected signal detected. Either atransceiver or separate transmitting and receiving antennae can be usedfor transmitting and receiving the microwave signal. As with thetransmission method the attenuation and phase shift of the reflectedsignal are measured and preferably are correlated with wt % unburntcarbon using least squares regression and equations of the same form as(13), (14) and (15).

FIG. 1 schematically shows the arrangement of the apparatus to measureunburnt carbon content of fly ash according to this invention. As shownthe apparatus comprises a microwave source which takes the form of aYttrium-Iron-Garnet oscillator 1 tuneable over the range 2 to 4 GHz andcontrolled by a data logging computer 2. The output of oscillator 1 ismodulated by a PIN diode modulator 3 and directed through a low passfilter 4 to a power divider 5. Power divider 5 diverts a small amount ofthe microwave signal to an 8-port junction 6 as a reference signal. Theremainder of the microwave signal is directed via a circulator 7 to atransmitter antenna 8. Circulator 7 is provided to direct any reflectedsignal to an appropriate instrumentation amplifier 9 to provide ameasurement signal for computer 2. Transmitter antenna 8 directs themicrowave signal through a sample measurement chamber 10 to a receiverantenna 11 from which the received signal is directed to 8-port junction6 and instrumentation amplifiers 9 to provide a measure of theattenuation and phaseshift of the received signal in the known manner.This data is transmitted for processing in the manner described herein.

The microwave antennae can be of any type suitable to the selectedsample presentation technique. FIGS. 2 to 4 show three preferredarrangements of the antennae and sample measurement chamber.

Referring to FIG. 2, an arrangement for measuring an ash sample in freespace is provided. The antennae are horn antennae 12, 13 and the ashsample 14 is contained in a container 15 formed of a material such aswood or plastic which allows the transmission of microwaves. In thisarrangement the ash sample 14 is packed in container 15 and suitablypositioned between horns 12, 13. The phase shift and attenuation aredetermined as described above and used to calculate the wt % of unburntcarbon as described above.

FIG. 3 shows an arrangement for measurement on sample in a waveguide. Inthis arrangement the antennae are capacitive posts or inductive loops16, 17. The sample 14 to be measured is packed into a section ofwaveguide 18 of circular or rectangular cross section suited to thefrequency range of the microwave signal. For measurements in the 2.6 to3.95 GHz frequency range an RG-48 rectangular waveguide can be used. Thesample is confined to the selected region of the waveguide by plasticsheets 19 which allow transmission of the microwave signal. The phaseshift and attenuation are determined as described above and used tocalculate the wt % of unburnt carbon as described above.

FIG. 4 shows an arrangement for measurement on a sample in a microwaveresonant cavity. In this arrangement the ash sample is contained in anon-conducting, for example, ceramic or plastic tube 20 located alongthe axis of a TE or TM mode resonant cavity 21. The microwave signal iscoupled in and out of the resonant cavity using H-field (inductive loop)probes 22, 23.

FIG. 5 shows another arrangement for the measurement of the unburntcarbon content of a fly ash sample. A variable frequency microwaveoscillator 30 provides a microwave signal to a microwave power divider31. Power divider 31 produces two output signals which are respectivelydirected to a reference microwave resonant cavity 32 and a measurementmicrowave resonant cavity 33. The fly ash sample (not shown) is placedin or appropriately passed through the measurement cavity 33. Detectors34 and 35 respectively measure the attenuation of the microwave signalrespectively propogated in the reference cavity and measurement cavity.Detectors 34 and 35 can be of any suitable known type such as diodedetectors. The outputs of detectors 34, 35 are fed to a processor 36which is used to determine a measure of the resonant frequency,transmitted power at the resonant frequency, and Q-factor of bothcavities from the swept frequency response (i.e. attenuation) of thereceived signal. A measure of weight percent unburnt carbon can then beprovided by the processor as explained by the following.

If the resonant frequency and Q-factor of the reference cavity are f_(r)and Q_(r) respectively, and the resonant frequency and Q-factor of themeasurement cavity are f_(m) and Q_(m) respectively, then the weightpercent unburnt carbon in the fly ash is determined of the sample bulkdensity from a function of the form ##EQU7##

    where,Δf=f.sub.r -f.sub.m                            (17) ##EQU8## Typically ##EQU9## is a correlation function of the form ##EQU10## where a.sub.o, a.sub.1,a.sub.2 . . . . are fitting constants or, ##EQU11## where b.sub.o, b.sub.1,b.sub.2 . . . . are fitting constants

The significant advantages of this arrangement compared to that using asingle measurement in a microwave resonator are that the measured Δf andΔ(1/Q) are effectively independent of drifts in the microwave oscillatoroutput frequency due to ambient temperature variations or drift in theoscillator control voltage. This is a consequence of the measurementperiod of the frequency sweep being much less than the period over whichsuch drifts normally occur. Therefore using this technique highmeasurement accuracy can be achieved without the need for a highlystabilized microwave source or electronics. This enables Δf to bedetermined from measurement of ΔV, the difference in the control voltageof the microwave oscillator at f_(m) and f_(r), rather than from themore difficult and expensive technique of using a microwave frequencycounter.

The measured Δf and Δ(1/Q) are independent of temperature drift in themicrowave detectors, as such drift only effects the amplitude of thedetected microwave signal. If the reference and measurement cavities aresubstantially similar in design and dimension the measured Δf and Δ(1/Q)are also independent of drifts in the resonant frequency of the cavitiesdue to metal expansion with ambient temperature variations. In thisarrangement it is desirable to place a standard absorber in thereference cavity such that f_(r) is just greater than the maximum f_(m)that occurs in the particular measurement application. In this case theswept frequency range, Δf, is minimised.

When the method described above is performed using a single microwavecavity a reference measurement is made when the cavity does not containthe sample. Preferably the period between reference measurements issubstantially shorter than oscillator, electronic and temperaturedrifts.

The apparatus described with reference to FIGS. 1 and 2, FIGS. 1 and 3,and FIGS. 1 and 4 respectively were used to perform measurements on arange of fly ash samples from New South Wales and Queensland powerstations. The unburnt carbon content of these samples was determined bystandard chemical analysis using LECO analyser and was in the range 0.5to 13 wt %. For measurement, in free space and in waveguides the sampleswere packed in an open container to a depth of approximately 100 mm, andin a 200 mm length of RG-48 waveguide section respectively, and thephase shift and attenuation of a 3.3 GHz microwave signal determined.The data were correlated with wt % carbon using the equations,

    wt % carbon=a.sub.o +a.sub.1 (φ.sub.fly ash /w)        (21)

    wt % carbon=b.sub.o +b.sub.1 (A.sub.fly ash /w)            (22)

where a_(o) , . . , b₁ are fitting constant, w is sample mass per unitarea (in g cm⁻²) and φ_(fly) ash and A_(fly) ash are the phase shift (indegrees) and attenuation (in dB) of the fly ash sample respectively.

The apparatus described with reference to FIG. 5 was also used toperform measurements on one of the fly ash samples. In this case thedata were correlated with wt % carbon using equation 19.

R.m.s. errors from correlations on the data using equations (12) and(13) are given below in Table 1.

                  TABLE 1                                                         ______________________________________                                                                   R.m.s. (wt %                                               Unburnt  Measure-  Error  Carbon)                                                                              Equa-                                Power   Carbon   ment      Equation                                                                             Equation                                                                             tion                                 Station (Wt %)   Geometry  (21)   (22)   (19)                                 ______________________________________                                        Waller-  3-13    Free space                                                                              0.41   1.41   --                                   awang            Waveguide 0.28   1.22   --                                   Swanbank                                                                              0.5-5    Free space                                                                              0.17   0.83   --                                                    Waveguide 0.22   0.70   --                                                    Resonator --     --     0.34                                 Eraring 0.5-2.5  Waveguide 0.19   0.29   --                                   ______________________________________                                    

Plots of the data for Swanbank fly ash samples are presented in FIGS. 6and 7 for measurements in waveguide and FIGS. 8 and 9 for measurementsin free space and FIGS. 10 and 11 for measurements in a resonator. Ther.m.s. errors in Table 1 represent the total analysis error due to gaugeinaccuracy, sampling and chemical analysis. These results indicate thata measurement of phase shift or the resonator characteristics is themost accurate for the determination of carbon content, and the accuracyof analysis is comparable to or better than that obtained with previousmethods.

The apparatus described above is particularly suitable for on-lineanalysis of the unburnt carbon content of fly ash sampled from a boileroutlet duct. Fly ash is removed from the boiler outlet duct byconventional sampling means (not shown), for example using a Cegritsample and cyclone, and passed through the sample measurement chamber ofthe apparatus. The fly ash can be fed continuously or in batches, andcarried to and from the measurement chamber by any suitable means, forexample by a screw conveyor.

The foregoing describes the invention with reference to some specificexamples and it will be apparent to those skilled in the art thatmodifications can be made without departing from the scope of theinvention.

I claim:
 1. An apparatus to measure the unburnt carbon content of a flyash sample comprising:a reference chamber; a measurement chamber formeasurement of the fly ash sample; means to generate a microwave signal;transmitter means to launch the microwave signal for transmissionthrough the reference chamber and the fly ash sample; receiver means toreceive a signal passed through the sample; and processing means fordetermining the attenuation and phase shift of the signal passed throughthe sample and the reference chamber with respect to the launched signaland for producing a measure of unburnt carbon content.
 2. An apparatusto measure the unburnt carbon content of a fly ash samplecomprising:means to generate a microwave signal; antennae means tolaunch the microwave signal into the fly ash sample, and to receive areflected signal; and processing means for determining the attenuationand phase shift of the reflected signal with respect to the launchedsignal, and for measuring the unburnt carbon content of the fly ash, andmeans for determining a reference measurement of the attenuation andphase shift without the fly ash sample.
 3. The apparatus as claimed inclaim 1 wherein at least one of the transmitter means and the receivermeans include antenna.
 4. The apparatus as claimed in claim 2 whereinthe antennae means include a single antenna for both launching andreceiving the microwave signal.
 5. The apparatus as claimed in claim 3,wherein the measurement chamber contains the fly ash sample.
 6. Theapparatus as claimed in claim 5 wherein the antenna include horn antennaand the measurement chamber includes a material permitting transmissionof microwaves.
 7. The apparatus as claimed in claim 5 wherein theantenna include inductive loop antenna and the measurement chamber is asection of waveguide.
 8. The apparatus as claimed in claim 5 wherein theantenna include inductive loop antenna and the measurement chamber is asection of waveguide.
 9. The apparatus as claimed in claim 2 wherein theantenna means include inductive loops.
 10. The apparatus as claimed inclaim 2 wherein the antenna means include capacitive post antenna. 11.The apparatus as claimed in claim 5 wherein the measurement chamberincludes a microwave resonant cavity.
 12. The apparatus as claimed inclaim 11 wherein the microwave resonant cavity operates in a TE mode.13. The apparatus as claimed in claim 11 wherein the microwave resonantcavity operates in a TM mode.
 14. The apparatus as claimed in claim 11wherein the fly ash sample is disposed about an axis of the microwaveresonant cavity.
 15. The apparatus as claimed in claim 2 wherein themicrowave signal is launched and received by a microwave transceiver.16. The apparatus as claimed in claim 15 further comprising a microwavereflector disposed on the distal side of the fly ash sample to theantenna means to reflect the microwave signal passed through the fly ashsample back through the sample to the antenna means.
 17. The apparatusas claimed in claim 1 wherein the measurement chamber includes amicrowave resonant cavity, and the processing means determines a changein the characteristics of the resonant cavity induced by the fly ashsample and produces therefrom the measure of unburnt carbon content. 18.The apparatus as claimed in claim 17 wherein the change in thecharacteristics are determined by comparing the signal received by thereceiver means with a reference signal obtained from a measurement ofthe reference chamber not containing the sample.
 19. A method ofmeasuring the unburnt carbon content of a fly ash sample comprising thesteps of:launching a microwave signal into a reference chamber and ameasurement chamber containing the fly ash sample, receiving themicrowave signal; determining the attenuation and phase shift of thereceived signal with respect to the launched signal; and producing ameasure of unburnt carbon content from the attenuation and phase shift.20. A method of measuring the unburnt carbon content of a fly ash samplecomprising the steps of:launching a microwave signal into the fly ashsample; receiving a component of the signal reflected from the fly ashsample; determining the attenuation and phase shift of the reflectedsignal with respect to the launched signal; correlating a measure ofunburnt carbon content with the attenuation and phase shift; anddetermining a reference measurement of the attenuation and phase shiftwithout the fly ash sample.
 21. The method as claimed in claim 19including transmitting the microwave signal into the fly ash sample infree space.
 22. The method as claimed in claim 19 including disposingthe fly ash sample in a waveguide.
 23. The method as claimed in claim 19including disposing the fly ash sample in a microwave resonant cavity.24. The method as claimed in claim 23 including operating the microwaveresonant cavity in a TE mode.
 25. The method as claimed in claim 23including operating the microwave resonant cavity in a TM mode.
 26. Themethod as claimed in claim 23 including determining the measure of theunburnt carbon content of the fly ash sample from a change in thecharacteristics of the microwave resonant cavity induced by the fly ashsample.
 27. The method as claimed in claim 26 including determining thechange in the characteristics of the microwave resonant cavity bycomparing the received signal with a reference signal passed through thereference chamber.
 28. The apparatus as claimed in claim 1 wherein thetransmitter means and the receiver means include antenna.
 29. Theapparatus as claimed in claim 1 wherein the processing means determinesthe change in the characteristics of the resonant cavity induced by thefly ash sample and produces therefrom the measure of unburnt carboncontent.
 30. The method as claimed in claim 15 including transmittingthe microwave signal into the fly ash sample in free space.